The biology of pneumocystis is being investigated to determine the role of host defense mechanisms is providing host protection. A) Antibody-Parasite Interacation. Pneumocystis trophozoites have been purified, disrupted, and assessed in a variety of columns. Specific protein bands for pneumocystis have been identified. By Western Blot technique, rat and human sera from normal and diseased individuals have been assessed for specific binding to these raised and shown to interact with this specific protein. Use of the monoclonal antibodies has shown differences between human and rat pneumocystis that may have important implications for the development of diagnostic tests. These differences are being studied. B) New Therapeutic Agents. In collaboration with DCT, dihydrofolate reductase (DHFR) has been isolated from pneumocystis and toxoplasma, the first time enzyme has been recovered from these protozoa. A new DHFR inhibitor has been identified which is much more active than previously active drugs. In vitro and animal studies show to be at least as effective as older drugs. Systematic studies of sulfones and sulfonamides are being performed using sophisticated enzymology techniques. The significance of these studies is that they provide new insights into diagnosis and therapy of this important opportunistic pathogen.